1. Field of the Invention
The present invention relates to an inkjet printing apparatus and a determination method of a driving pulse applied to an inkjet printing.
2. Description of the Related Art
A printing apparatus, which adopts an inkjet printing system, forms an image by repeating an operation for scanning a printhead having orifices used to discharge an ink with respect to a printing medium, and an operation for conveying the printing medium in a direction perpendicular to the printhead scanning direction. Such printhead includes heat generation elements (heaters) for generating a thermal energy upon application of driving pulses.
In a printing apparatus of a type which discharges an ink using such heaters, the ink is film-boiled using the thermal energy generated by applying driving pulses to the heaters, and is discharged using pressures of bubbles generated at that time. For this reason, as a printing operation progresses, the temperature of the printhead gradually increases. Since ink viscosity decreases with increasing temperature, an ink discharge amount changes if the printing operation is continued under the same condition.
For this reason, control is executed to change a head driving pulse signal according to a change in temperature of a substrate (to be also referred to as a head temperature hereinafter) so as to maintain a constant ink discharge amount. Japanese Patent Laid-Open No. 05-31905 discloses a technique for selecting a pulse width of a driving pulse to be applied to the heaters according to a temperature measured by a temperature sensor included in the printhead.
In this case, upon execution of printing while reducing the number of used nozzles, that is, the number of used heaters, when the pulse width of the driving pulse is changed, as described in Japanese Patent Laid-Open No. 05-31905, it is revealed that the following harmful effects occur.
FIGS. 24A and 24B are graphs prepared by plotting lightness levels on a printing medium when the pulse width of the driving pulse is changed. The ordinate plots lightness levels, and the abscissa plots the position of the printing medium. FIG. 24A shows a change in lightness when the printing operation is executed using 768 nozzles. FIG. 24B shows a change in lightness when the printing operation is executed using 1024 nozzles (all nozzles).
With reference to these results, as can be seen from FIG. 24B, a change in lightness after the pulse width of the driving pulse is changed is moderate when the number of used nozzles is larger. On the other hand, as can be seen from FIG. 24A, a change in lightness after the pulse width of the driving pulse is changed is sharp when the number of used nozzles is reduced.
This is because when the number of used nozzles is reduced, a conveyance amount of the printing medium per scan of the printhead is decreased, and a change in ink discharge amount occurs within a narrow area on the printing medium. When a change in lightness occurs within a narrow area on the printing medium, the user visually confirms it as uneven density on the printing medium.
The present invention has been made in consideration of the aforementioned problems, and has as its object to provide a technique for obscuring uneven density to be visually confirmed.